Servoactuated hydraulic transducer apparatus

ABSTRACT

Servoactuated hydraulic transducer apparatus of axial piston type having a rotatable piston barrel mounted within a body, rocker cam defining a swash plate mounted in pivotable fashion within a cam cradle defined within the body, and a plurality of pistons axially mounted within the piston barrel and abutting the swash plate. The pistons are adapted for reciprocation upon rotation of the barrel relative to the swash plate. Two actuators are linked in opposition with the rocker cam to pivot the rocker cam from a position causing maximum fluid displacement of the pistons in a first direction of flow through a position of minimum fluid displacement to a position of maximum fluid displacement in a second and opposite direction of flow. The actuators are respectively connected to receive fluid actuating pressure through a servo selector valve. The valve includes an externally actuated control spool valve means adapted to direct fluid pressure respectively to one actuator or the other. The valve also includes a follower sleeve valve means disposed around the spool valve means and mechanically linked with the rocker cam. The follower valve means is adapted to close off fluid flow through the control valve means when the rocker cam has been pivoted to a position commensurate with a respective position established for the control valve means. The geometry of the mechanical linkage is such that the response of the swash plate to the control spool valve means is essentially linear. The control valve means may be actuated directly or actuated remotely through a hydrostatic electrical control system.

t lnited States Patent Knapp 1 Apr. 16, 1974 SERVOACTUATED HYDRAULIC TRANSDUCER APPARATUS [75] Inventor: Kenneth H. Knapp, l-lillsdale, Mich.

[73] Assignee: Abex Corporation, New York, NY.

[22] Filed: Nov. 14, 1972 [21] Appl. No.: 306,503

[52] US. Cl. 91/506 [51] Int. Cl. F01b 13/04, F04b 49/00 [58] Field of Search 60/440; 417/222; 91/506 [56] References Cited UNITED STATES PATENTS 3,606,755 3/1970 Connett 60/446 3,669,570 6/19 72' Himmler 4'17/222 2,945,449 7/1960 'Le Febvre et a1. 91/506 3,164,960 1/1965 Weiserbach et al.. 417/222 3,257,959 6/1966 8111111 1 4l7 /217 3,406,608 10/1968 Diedrich ..91/506 3,733,963 5/1973 Kubilos 91/506 FOREIGN PATENTS OR APPLICATIONS 212,927 5/1959 Great Britain..... 91/506 1,243,776 7/1971 Great Britain 91/506 Primary Examiner-William L. Freeh Attorney, Agent, or Firm-Thomas S. Baker, Jr.; David A. Greenlee [57 ABSTRACT Servoactuated hydraulic transducer apparatus of axial piston type having a rotatable piston barrel mounted within a body, rocker cam defining a swash plate mounted in pivotable fashion within a cam cradle defined within the body, and a plurality of pistons axially .direction of flow through a position of minimum fluid displacement to a position of maximum fluid displacement in a second and opposite direction of flow. The actuators are respectively connected to receive fluid actuating pressure through a servo selector valve. The valve includes an externally actuated control spool valve means adapted to direct fluid pressure respectively to one actuator or the other. The valve also includes a follower sleeve valve means disposed around the spool valve means and mechanically linked with the rocker cam. The follower valve means is adapted to close off fluid flow through the control valve means when the rocker cam has beenpivoted to a position commensurate with a respective position established for the control valve means. The geometry of the mechanical linkage is such that the response of the swash plate to the control spool valve means is essentially linear. The control valve means may be actuated directly or actuated remotely through a hydrostatic electrical control system.

5 Claims, 9 Drawing Figures SERVOACTUATED HYDRAULIC TRANSDUCER APPARATUS BACKGROUND OF THE INVENTION This invention generally relates to fluid flow transducer apparatus such as an axial piston pump or motor and more particularly relates to such a pump or motor of variable fluid displacement having its displacement established by a directly or remotely actuated hydraulic pressure actuator system. Additionally, the inventive apparatus may include a remotely controlled self equalizing hydrostatic actuator control apparatus.

Pumps, when used herein, shall mean both pump and motor hydraulic flow transducer apparatus. Axial piston pumps and motors including various control apparatus are well developed as shown in representative U.S. Pats, No. 2,749,844, N0. 2,835,288, No. 2,915,985, NO. 3,017,864, N0. 3,089,426, NO. 3,250,277, NO. 3,286,601, NO. 3,302,585, NO. 3,381,624, NO. 3,405,646, NO. 3,422,767, NO. 3,429,225, NO. 3,481,277, N0. 3,510,231, NO.

I 3,554,671, No. 3,588,286, and com only assigned and copending United States Patent applications Ser. No; 128,733, filed Mar. 29, 1971 and Ser. No. 165,063, filed July 22, 1971 These patents are herein referenced as additional background and information concerning the following description.

SUMMARY OF THE INVENTION The invention provides axial piston fluid flow transducer apparatus wherein the fluid displacement components and cooperating control components are provided as an integral unit of relatively simple construction for facility of installation, adjustment and maintenance.

This invention also provides axial piston fluid flow transducer apparatus wherein the fluid displacement may be controlled in infinite variation from a maximum displacement in one direction through zero displacement to a maximum displacement in the opposite direction. a

This invention further provides axial piston fluid flow transducer apparatus wherein the fluid displacement control system may be actuated remotely by improved hydrostatic control means for more versatile application and operation of the apparatus.

The foregoing provisions and other objects and advantages of the invention are attained by servoactuated transducer apparatus of axial piston type having a rotatable piston barrel mounted within a body, a rocker cam defining a swash plate mounted in pivotable fashion within a cam cradle defined within the body, and a plurality of pistons axially mounted within the piston barrel and abutting the swash plate. The pistons are adapted for reciprocation upon rotation of the barrel relative to the swash plate. Two actuators are linked in opposition with the rocker cam to pivot the rocker cam from a position causing maximum fluid displacement of the pistons in a first direction of flow through a position of minimum fluid displacement to a position of maximum fluid displacement in a second and opposite direction of flow. The actuators are respectively connected to receive fluid actuating pressure through a servo selector valve. The valve includes an externally actuated control valve means adapted to direct fluid pressure respectively to one actuator or the other. The

valve also includes a follower valve means mechani- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of a pump incorporating the present invention.

FIG. 2 is a partially cut away rear end elevation of the pump of FIG. 1.

FIG. 3 is a partial axial section of the pump taken along the line 3-3 of FIG. 2.

FIG. 4 is a section of the pump taken adong the line 44 of FIG. 3.

FIG. 5 is a partial section of the pump controller taken along the line 55 of FIG. 2.

FIG. 6 is a skeletal view of the control linkage of the pump.

FIG. 7 is'a section of the pump controller mechanism taken along the line 77 of FIG. 2 and additionally schematically illustrates a remotely connected hydrostatic actuator for the controller.

FIG. 8 is an enlarged partial section of the controller as shown in FIG. 7 and illustrating the controller actuated to cause a change in displacement of the pump.

FIG. 9 is a partial section of the controller as shown in FIG. 7 and illustrating the controller actuated to cause an opposite change in displacement of the pump.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 14, there is shown a hydraulic transducer pump or motor 900 of the axial piston type having a drive shaft 11 rotatably mounted through a port block 12 into a rotatable piston barrel 10, which in turn is rotatably mounted within a pump body 13 by means of a bearing assembly 14. A plurality of pump pistons 15 are mounted within respective cylinders 16 defined within the barrel 10. A piston shoe 17 is pivotally attached to each piston and retained against a creep plate 18 by means of a shoe retainer assembly 19. Creep plate 18 is also termed a swash plate herein. Creep plate 18 and retainer assembly 19 are mounted with a rocker cam 21 which is received in pivoted relation within a cradle 22 defined by an end cap 23. End cap 23 abuts a spacer ring 24 wiich supports bearing assembly 14 and barrel 10 into appropriate operating position when end cap 23 is assembled with body 13 as shown. End cap 23 is connected with body 13 by means of fasteners (not shown) such as cap screws. A bearing 25 is interposed between the complementary cylindrical surfaces of rocker cam 21 and cradle 22 to provide substantially facile pivoting movement of the rocker cam within the cradle. Bearing 25 is best shown in FIGS. 3 and 4.

Also shown in FIG. 4 are retainer brackets 26 which serve to retain rocker cam 21 within cradle 22 when pump 900 is not producing a discharge fluid pressure. As is well known, the discharge pressure produced by pump 900 urges rocker cam 21 into firm contact with bearing 25 when the pump is in operation.

As seen in FIG. 3, shoe retainer assembly 19 is comprised of shoe retainer plate 27 which retains piston shoes 17 against creep plate 18 during operation of the pump 900. Retainer plate 27 is supported from rocker cam 21 through a combination radial and thrust bearing 28 and a keyed shaft 29 which extends into a counterbore 31 of the rocker cam as shown. Shaft 29 is resiliently secured in rocker cam 21 by means of a shouldered nut 32 which supports and holds spring means 33 (Belleville springs, for example) in compression. It is to be noted that the shoulder of nut 32 slightly clears the counterbore 31 to enable retainer plate 27 to move very slightly responsive to axial force applied along shaft 29. The purpose of this feature as shown and described is to allow only limited lift-off of piston shoes 17. Such lift-off behavior is well known and further apparent from the prior art herein referenced.

Further referring to FIG. 3, the inner end of drive shaft 11 is seen to be secured in splined connection within barrel by means of a thrust snap ring 34 which retains the shaft in the inner axial direction as shown and which is secured by a fastener such as a cap screw 35 drawn down against a spring means 36 (such as Bellville springs) and a support washer 37, the washer 37 being in abuttment with the barrel 10 as shown.

As best shown in FIGS. 3 and 4, counterbore 31 opens into a bore 38 which forms a socket which houses the base of a control arm 39. Control arm 39 is retained within bore 38 by means ofa snap ring 41. The distal end of control arm 39 defines a fork 42 which receives a control arm pin 43 as shown.

End cap 23 defines opposed cylinders 44 and 45 which receive a double ended piston 46. The center of piston 46 is cut away and bored toreceive control arm pin 43 and fork 42 as shown. Suitable seals such as 0- rings are provided at each end of piston 46. The ends of cylinders 44 and 45 are closed by heads 47 and 48 which are attached to end cap 23 by means of fasteners (not shown) such as cap screws. With the construction as shown, the piston 46, cylinders 44 and 45 and heads 47 and 48 define chambers 49 and 51.

As partially and somewhat schematically shown in FIGS. 2 and 3, end cap 23 and heads 47 and 48 define fluid passages 52 and 53 which are in respective communication with passages 54 and 55 which are defined in a servo control unit 56. Continuation of passages 54 and 55 is shown in FIGS. 79.

The servo control unit 56 and its associated mechanical linkage with rocker cam 21 are best illustrated in FIGS. 4-7, the linkage as such being best shown in FIG. 6.

As previously mentioned, rocker cam 21 is retained within cradle 22 by means of retainer brackets 26. Brackets 26 define arcuate slots (FIG. 4) through which dowell pins and 30 extend from the cam member as shown. As provided, the pins are adapted to move through an arcuate path as the rocker cam is pivoted about within cradle 22.

As seen in FIGS. 4-6, dowell pin 30 is pivotally linked with a feedback lever 95 which is fixed to a control shaft 92 by means of a key 96. Shaft 92 is mounted through a bearing 93 and extends into body 57 of control unit 56. A feedback yoke 91 is attached to the distal end of shaft by means ofa key 94. The divided ends of yoke 91 straddle the midsection of a follower valve sleeve 61 and fit into a circumferential groove defined in the sleeve in a manner causing axial movement of the sleeve within control body 57 when the shaft 92 is rotated through lever 95 by arcuate movement of dowell pin 30.

It is to be noted that displacements of fluid into and out of chambers 49 and 51 causes commensurate arcuate movement of rocker cam 21. Movement of rocker cam 21 causes corresponding movement of dowell pin 30 which movement is transferred through feedback lever 95 and shaft 92. Rotation of shaft 92 pivots yoke 91. Movement of yoke 91 causes corresponding axial movement of follower sleeve 61.

The entire mechanical feedback linkage system is best shown in FIG. 6. It is to be noted that the geometry of the mechanical linkage is to be such that the response of the swash plate in establishing fluid displacement rate to the setting of the control valve spool 62 is essentially linear.

Referring now to FIGS. 5 and 7 (and corresponding FIGS. 8 and 9), control unit 56 includes a body 57 having a side cover 58 and an upper end cover 59. Body 57 is seen to define a bore which receives follower valve 61 in reciprocative and fluid tight relation. A hollow control valve spool or spool valve 62 is received in reciprocative and fluid tight relation within follower sleeve 61.

As shown in FIG. 7, the passages 54 and 55 which, communicate with the piston displacement chambers 49 and 51 of FIG. 3. Body 57 also defines passages 82, 83, 84, and 85 as shown. Passage 82 is located above passage 54 and is in communication to receive an incompressible fluid, under pressure from a source P, which fluid serves to actuate the piston 46 as dictated by control unit 56 as later described. The passage 83 is located between passages 54 and 55 and serves as an exhaust passage for displacement of fluid from chamber 49 or chamber 51 as dictated by operation of control unit 56. Passages 84 and 85 are in communication with passage 83 as shown and serve to provide equal and opposite balancing forces to the ends of sleeve 61 and spool 62. Fluid under pressure is also supplied to a piston and cylinder control actuator arrangement mounted to the lower end of body 57 and later described. The fluid exhausted into passage 83 is transmitted back to the case chamber 86 (FIG. 3) through a back pressure valve (not shown) which maintains the exhaust pressure in passage 83 at about 2580 psi, for example, to serve as a proper supply pressure to actuator 70.

The sleeve 61 defines four circumferential grooves as shown with ports defined to the interior of the sleeve at the bottom of the grooves. Spool 62 also defines three circumferential grooves with ports defined in the upper and lower grooves for communication through the hollow passage 87. The open end of spool 62 is closed by a threaded plug or the like as shown.

The grooves (and adjacent lands) of spool 62 are spaced with respect to the ports in sleeve 61 to: (a) close off the ports to passage 87 and to close ofi the sleeve ports to passages 54 and 55 when sleeve 61 is positioned relative to spool 62 as shown in FIG. 7; (b) provide communication between passages 82 and 54 and between passages 55 and 83 when spool 62 has been moved downwardly with respect to sleeve 61 as shown in FIG. 8; and (0) provide communication through passages 82, 87 and 55 and between passages 54 and 83 when spool 62 has been moved downwardly with respect to sleeve 61 as shown in FIG. 9.

Actuator 70 comprises a cylinder 63 mounted with body 57 as shown and closed at its lower end by an end.

cap 64. An internally threaded end cover 65 is attached to cap 64. Cylinder 63 receives a slidable piston unit 66 having shafts extending above and below the cylinder. The upper shaft is in connection with the lower end of valve spool 62 through a lateral undercut slot retainer arrangement as shown, such connection being maintained by an axially compressed spring 72.

The lower shaft or piston 66 is connected into a centering spring arrangement, a retainer ring 71, a slightly biased spring 67, and a lower retainer ring 73, all disposed around the lower shaft and retained as shown by means of a washer 74 and a screw 75. The retainer rings and spring are received into a bushing 69 which is adjustably threaded into cover 65. A spring snap ring 71 maintains the retainer rings, spring and shaft in centered relation within the bushing 69 with the spring 67 being further compressed if the shaft is moved in either direction relative tothe bushing. Adjustment of the bushing 69 establishes a' selected rest position for piston 66 and for valve spool 62, which establishes a zero or preselected displacement position for swash plate 18 as will become more apparent. Adjustment of bushing is fixed by jam or locknut 76. A plug 77 closes off the lower end of bushing 69 as shown.

Piston unit 66 defines a chamber 97 and a chamber 98 on either side of the piston within cylinder 63. Chamber 97 has fluid communication through a conduit 101 to a remote actuator unit 100. and chamber 98 has fluid communication through a conduit 99 to the remote actuator. I Fluid under pressure is communicated from passage 84 through a make-up passage 88 into passage 89 as shown and from passage 89 respectively past ball check valves 78 into conduits 99 and 101. The ball check valves are shown biased into closed position by springs 79 retained by retainer plug 81. With the arrangement shown fluid will flow through a respective check valve into chambers 97 and 98 and to actuator unit 100, should the internal pressure in these elements fall below the pressure in passages 83, 84, and 85.

In FIG. 7, remote actuator 100 is seen to comprise a body 102 defining a bore which receives a control spool 110 in reciprocative relation. An end cap 103 closes one end of body 102 and a control lever cap 104 closes its other end. Retainer rings 107 and 108 are held in abuttment with shoulders on either end of spool 110 by biased centering spring 105 and 106 to urge spool 110 into centered and neutral position. One end of spool 110 extends into a closed chamber 113 and the other end of the spool extends into a closed chamber 114 as shown. A passage 118 is defined by body 102 which connecs chamber 114 with conduit 101 and a passage 119 is also defined which connects chamber 113 with conduit 99. I

Control lever cap 104 defines a lateral bore which receives a rotatable operating shaft 115. Shaft 115 is linked to spool 110 through an operating arm as shown to move the spool in either direction through rotation of the shaft. A hand operating lever 116 is connected to shaft 115 outside cap 104 to enable spool 110 to be manipulated to displace fluid from either chamber 113 or 114 as desired to actuatecontrol 70.

A narrow circumferential slot 112 is defined around the center of spool 110 which slot is adapted to be in communication with-small passages 109 and 111 defined in body 102 as shown. The arrangement is such, that when lever 116 is released, the spool returns to its centered position with conduit 109 in communication with conduit 111 through slot 112. When thus centered, the system of chambers 97, 98, 113 and 114 is in equilibrium with valves 78 making up any shortage of fluid lost by leakage or the like. Venting valves (not shown) are provided in the system to purge compressible fluids such as air.

Though the control unit is described as hydraulic, there may also be provided manually or electrically op erated units. The hydraulic control unit as described is suitable for mining machinery, for example. A manual unit may be useful at times. An electrical unit may be useful in connection with automated control systems. For example, modifications of the electrical control units disclosed in U.S. Pat. No. 3,401,711 or No. 3,424,183 may be adapted.

OPERATION OF THE PREFERRED EMBODIMENT Operation of the control unit 70 through operation of the remote actuator is readily apparent. Variation of the fluid displacement of pump 900 through positioning of central spool 62 will become more apparent through reference to FIGS. 8 and 9 taken in view of FIGS. 3-7.

The yoke 91 and sleeve 61 are shown in centered position in FIGS. 8 and 9 which, through the linkage of FIG. 6, places the swash plate 18 at right angles to the axis of the pump with corresponding zero fluid displacement through the pump. In FIG. 8, the control spool has been moved down as shown, but movement of piston 46, rocker cam 21 and yoke 91 through the associated linkage has not yet begun. Fluid from source P starts into piston chamber 49 through passages 82, 54, and-52 and starts to displace fluid from chamber 51 through passages 53, 55, and 83. In response, the piston 46 and rocker control arm 39 begin to move down, pivoting swash plate 18. Through the linkage of FIG. 6, the movement of rocker cam 21 causes corresponding movement of yoke 91 and downward movement of sleeve 61. The arrow at the right indicates the direction of motion of the linkage. As sleeve 61 moves down, the flow through the various ports is reduced and stops when sleeve 61 has been moved through the linkage to a position established by control spool 62. The swash plate 18 then remains fixed by piston 46 until another displacement position is established by spool 62 in response to actuator unit 100. When the hand lever 116 of unit 100 is released, all linkage reverts to a neutral or other preselected position. The operation shown in FIG. 9 is similar but in reverse to the operation shown in FIG. 8.

In summary, the control consists of a means to position the input member (spool) porting to direct fluid under pressure to the appropriate passages, actuating means to vary the'pump swash plate angle, feedback linkage to communicate that swash plate position back to the feedback member (sleeve) which equalizes the control pressure on each side of the actuator, preventing further movement of the pump swash plate. Therefore, for each position of the input spool within the opcrating range, subject to mechanical restraints, there is one and only one position of the pump swash plate.

Under normal conditions, at a given spool position, the swash plate positions the sleeve through the feedback linkage to hold the swash plate in that position. Any disturbance unbalances these pressures at the ends of the pistons 46, causing corrective motion of the swash plate back to the established position.

When the spool is displaced, the change in port areas unbalance the pressure conditions at chambers 49 and 51 which causes the swash plate to move until the sleeve is positioned to again balance the pressures at the new position. Therefore, for any operating point free of mechanical restraints such as stops, the swash plate angle, and thereby the pump displacement, is controlled directly by the position of the input spool. The port configuration and anti-backlash feedback linkage may be designed to maintain this relationship to within very accurate limits 2 percent rated full angle with pilot pressures from source P above 400 psi). This type of follow-up servo is generally very accurate and stable and can be made to be fast acting if the application requires.

Since the position of the servo spool 62 dictates the pump displacement, the method of controlling this spool is very important. The method described herein may be called a liquid stick" and is based on the displacement of an incompressible fluid from one cylinder to another to cause motion of a piston.

Actuator unit 100 is the command or sender unit and the cylinder/piston arrangement 70 is the receiver unit. The replenishing check valves 78 of the receiver permit filling of the cavities and lines connecting sender and receiver. The sender has bleed plugs (not shown) to permit bleeding off air to insure incompressible fluid in the system. The return pressure used for replenishing must be sufficiently high to open the check valves to maintain this incompressible fluid condition (25-80 psi for example) in order to make up any leakage which may occur.

As the spool 110 is moved, it displaces fluid from one end chamber through connecting lines into an end cavity of the receiver. Since the fluid is substantially incompressible, this causes the piston 78 of the receiver 70 to move an amount proportional to the volume displaced. Preferably, the areas are equal so the receiver would move the same amount as the sender. As the receiver piston moves, it, of course, pushes the fluid from the opposite side back through the connecting lines into the following side of the sender unit. If at any time the pressure in either side of the trapped control volume drops below the return pressure in the servo, the checks open, replenishing the control fluid.-

The receiver piston 66 is connected through an antibacklash joint to the servo input spool 62. Therefore, the position of the pump swash plate 18 is controlled remotely by the position of the sender unit piston.

There are other features of note. There is a neutral position built into the actuator unit 100 in which the end chambers are interconnected through the narrow slot 112 of spool 110. At approximately 10.08 inch travel, for example, this porting is closed and the effective stroke begins.

The foregoing description and drawings will suggest other embodiments and variations to those skilled in the art, all of which are intended to be included in the spirit of the invention as herein set forth.

That being claimed is:

l. A hydraulic transducer comprising: a body; a rotatable barrel mounted within the body; a plurality of cylinders in the barrel; a rocker cam cradle in one end of the body; a rocker cam mounted on the rocker cam cradle and pivotable relative to the cradle; a creep plate attached to the rocker cam; a plurality of pistons attached to the creep plate and operable to be reciprocated within the cylinders when the barrel is .rotated; piston means linked to the rocker cam for pivoting the rocker cam from a position causing minimum fluid displacement through said transducer to a position causing maximum fluid displacement through said transducer; valve means for controlling the flow of fluid to operate the piston means; control means for actuating said valve means; second piston means for operating the valve means and connected to the control means; feedback linkage attached to the rocker cam and the valve means which operates the valve means to a position which prevents further movement of the first said piston means when the rocker cam is in the position designated by the control means; means for biasing the second piston means to a centered position to cause said valve means to permit fluid to move the first said piston. means to a position which pivots the rocker cam to cause minimum fluid displacement through the transducer; third piston means in said control means; means for biasing said third piston means to a centered position; wherein said third piston means and said second piston means are hydraulically connected; and means for balancing the fluid pressure at each end of the second piston means each time the third piston means is centered to thereby enable the biasing means to center the second piston means.

2. A hydraulic transducer as recited in claim 1; wherein said balancing means includes a slot in the third piston means and the slot is aligned with passages leading to each end of the second piston means when the third piston means is centered.

3. A hydraulic transducer as receited in claim 1; wherein movement of said third piston means causes a proportional movement of the second piston means which operates the valve means to permit fluid to move the first said piston means to cause the rocker cam to pivot to a position causing fluid displacement through the transducer; and the position of the rocker cam and the amount of fluid displacement from the transducer is proportional to the amount the third piston means is moved.

4. A hydraulic transducer as recited in claim I; including replenishing means for maintaining the fluid pressure in the hydraulic connection between the seocnd piston means and the third piston means.

5. A hydraulic transducer as recited in claim 4; wherein said replenishing means supplies fluid at one pressure through check valves to each end of said second piston means and said valve means supplies fluid at a greater pressure to the first said piston means. 

1. A hydraulic transducer comprising: a body; a rotatable barrel mounted within the body; a plurality of cylinders in the barrel; a rocker cam cradle in one end of the body; a rocker cam mounted on the rocker cam cradle and pivotable relative to the cradle; a creep plate attached to the rocker cam; a plurality of pistons attached to the creep plate and operable to be reciprocated within the cylinders when the barrel is rotated; piston means linked to the rocker cam for pivoting the rocker cam from a position causing minimum fluid displacement through said transducer to a position causing maximum fluid displacement through said transducer; valve means for controlling the flow of fluid to operate the piston means; control means for actuating said valve means; second piston means for operating the valve means and connected to the control means; feedback linkage attached to the rocker cam and the valve means which operates the valve means to a position which prevents further movement of the first said piston means when the rocker cam is in the position designated by the control means; means for biasing the second piston means to a centered position to cause said valve means to permit fluid to move the first said piston means to a position which pivots the rocker cam to cause minimum fluid displacement through the transducer; third piston means in said control means; means for biasing said third piston means to a centered position; wherein said third piston means and said second piston means are hydraulically connected; and means for balancing the fluid pressure at each end of the second piston means each time the third piston means is centered to thereby enable the biasing means to center the second piston means.
 2. A hydraulic transducer as recited in claim 1; wherein said balancing means includes a slot in the third piston means and the slot is aligned with passages leading to each end of the second piston means when the third piston means is centered.
 3. A hydraulic transducer as receited in claim 1; wherein movement of said third piston means causes a proportional movement of the second piston means which operates the valve means to permit fluid to move the first said piston means to cause the Rocker cam to pivot to a position causing fluid displacement through the transducer; and the position of the rocker cam and the amount of fluid displacement from the transducer is proportional to the amount the third piston means is moved.
 4. A hydraulic transducer as recited in claim 1; including replenishing means for maintaining the fluid pressure in the hydraulic connection between the seocnd piston means and the third piston means.
 5. A hydraulic transducer as recited in claim 4; wherein said replenishing means supplies fluid at one pressure through check valves to each end of said second piston means and said valve means supplies fluid at a greater pressure to the first said piston means. 